The stars — those points of light at night and the blazing ball in the day sky (sun) — show a great range in their sizes, temperature, age and composition. How do we know? — as approaching any star would be instant death! Through the laws of physics over the past several centuries, we have been able to learn what makes stars shine, what stars are made of, how hot they are and how long they will shine. More recently, satellites have been launched into special orbits to study our sun without the interference of Earth’s atmosphere.

Let’s start with star basics: Stars are gaseous nuclear reactors that consume hydrogen in their cores.

Stars are so large and massive that their interior temperatures and densities make possible the fusion (merger) of hydrogen into helium, converting a small percentage of matter into energy by E = mass x c squared. This is what powers our sun and 80% of all stars. We have been able to determine star distances by the motion of the Earth about the sun, variable stars and comparison with similar stars. By sensitive photometers (light detectors), we can use star distances and their light intensities to determine the power or luminosities of stars.

Stars come in a variety of sizes: biggest are the supergiants (hundreds of times our sun’s width), then the giants (dozens of times our sun’s width), dwarfs (of which our sun is one — about a million miles across), white dwarfs (the size of rocky planets — about 1% of our sun’s size) and neutron stars (about the size of a large city).  

Do stars age? The more massive (more material), a star the faster they run through their lives. The heaviest stars shine for only a few million years while the small dwarf stars can shine for hundreds of billions of years. This compares with 10 billion years for our sun’s life. The white dwarfs (exhausted cores of dead stars) are cooling down over billions of years. Neutron stars don’t have a life time, except for their slowing spinning motion which causes radio emissions (detected with radio telescopes).

Our sun was formed from a large interstellar gas cloud, from which a number of stars condensed (a star cluster). This star cluster gradually spread out due to the rotation of our Milky Way galaxy. This happened over 4.5 billion years ago so we are unable to identify the brother and sister stars of our sun. As our sun formed, there was a surrounding disk of debris from which our solar system developed.

Our sun was dimmer at the start but now has reached it’s present power of 383 trillion trillion watts. (A trillion is a million million.) The biggest star known is UY Scuti, with a radius 1,700 times our sun, a red supergiant 9,500 light years away with 340,000 times our sun’s power. The smallest star known is EBLM JO555-57AB, about the size of the planet Saturn with 2 10,000ths of our sun’s power, about 8% of our sun’s mass at 630 light years away.

The stars shining in the night sky consist mostly of hydrogen, that was distributed across our galaxy from its origin. Spectroscopic analysis shows that the concentration of stars’ surfaces are mostly hydrogen, about 1 tenth of helium and a few percent of elements such as oxygen, carbon, nitrogen and a smaller portion of metals such as iron and sodium.

Can stars explode? A large majority of stars undergo a swelling of their outer layers, which they eject as gaseous shells, temporarily lit up by the white dwarf star within. The white dwarf slowly dims over many billions of years. Stars that have eight or more times the mass of our sun will develop an iron core that leads to a supernova explosion, the way that heavy elements are spread across our galaxy. This is how we get precious elements such as gold, platinum and silver. Reference: Astronomy Magazine, July 2020

Sky sights in the week ahead

Bright Jupiter and Saturn can be seen in the late evening southeastern sky. The brilliant planet Venus is our morning star in the eastern dawn. The evening moon is half full on July 27, offering good views of its craters along its lighted left edge with a telescope.

Bob Doyle is a retired science teacher at Frostburg State University who is available to talk to adult and student groups about matters related to his columns. Contact him at

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